Hmmm interesting, not your average looking algae.
How old is the tank? Are you using RO/DI water only? Have you tested for phosphates and nitrates?
For right now I would use a sand vacuum and siphon it out, IF THE SAND BED is deep and older, do not vacuum the sand. If it's a young sand bed have at it. Get your water tested for nutrients as mentioned above. Proceed with Phosban or a GFO product if indicated by your phosphate value.